We have previously described techniques for energy control in power conditioning units (inverters) which enable the construction of a dc-to-mains power converter which does not need to use electrolytic capacitors (see U.S. Ser. No. 12/160,743 and GB2,434,490A). We now describe alternative techniques for increasing the reliability of a solar inverter where electrolytic capacitors are employed. Such techniques are important because electrolytic capacitors are prone to failure, especially at high temperatures.
Electrolytic capacitors have a number of different failure modes, including going open circuit and going short circuit. Open circuit failure can be caused, for example, by loss of electrolyte, for example through vaporisation, and this may be preceded by a drop in capacitance and an increase in dissipation factor (tan δ) or the ESR-equivalent series resistance of the capacitor). Moreover increase in the ESR can cause an increase in local heating and hence electrolyte evaporation, causing a thermal runaway effect. A short circuit failure can be caused by, for example, local breakdown in a dielectric separator or oxide layer. Protection of a shunt capacitor bank is described in ‘Shunt Capacitor Bank Fundamentals and Protection’, Brunello, Kasztenny and Wester, 2003 Conference for Protective Relay Engineers—Texas A&M University, Apr. 8-10, 2003, College Station Tex.
Bearing in mind the above described failure modes, we will describe techniques to increase the reliability of solar inverters, in particular where electrolytic capacitors are employed. In embodiments these techniques are particularly useful in single stage power circuits for photovoltaic inverters, since these have fewer switching stages than multi-stage power circuits and can therefore have better efficiencies. More particularly, single stage photovoltaic inverters use a bank of electrolytic capacitors at the input, which enables energy storage during the zero crossing of the grid supply, and allows a stable input voltage which facilitates maximum power point tracking (MPPT).
Background prior art can be found in: EP1475882A2; GB2399465A; EP0642199A1; U.S. Pat. No. 6,094,129A and U.S. Pat. No. 5,859,772A.